Loss of cochlear neurons and hair cells is a major cause of sensorineural hearing loss. One approach to potential treatments is the transplantation of exogenous progenitor cells. Our strategy for developing cell based therapies for inner ear disorders is to study the reinnervation of a de-afferented organ of Corti. We take this approach because 1) only a single cell type, the spiral ganglion cell, must be replaced to see functional recovery; 2) partial regeneration of damaged tissue by cell replacement has been successful in animal models of other neurodegenerative diseases, such as Parkinson's, suggesting that neuronal circuits in the adult nervous system can be reconstituted if appropriate progenitor cells are introduced and 3) understanding the mechanisms underlying re-formation of neural connections to hair cells in the adult ear is important to any therapeutic approach to sensorineural hearing loss. The Specific Aims comprise four inter-related experiments to probe key variables likely to influence the success of hair cell reinnervation by transplanted neural progenitor cells. In Aim 1 the developmental expression by spiral ganglion neurons of several families of neuronal guidance molecules will be studied by immunohistochemistry. In Aim 2 we use de-afferented cochlear explants to study the reinnervation of hair cells by grafted progenitors, in vitro, by modifying characteristics of the donor cells (e.g. silencing expression of selected guidance molecules or controlling degree of differentiation) or the host environment (e.g. developmental age of the explant or etiology of the initial de-afferentation). In Aim 3 we build on the in vitro results to choose donor cells for in vivo transplantation into a chemically de-afferented gerbil ear, in which the sensory cells are intact but spiral ganglion cells are destroyed. In this model system, success of reinnervation can be assayed both functionally and histologically. In Aim 4 we use a reporter assay to determine conditions needed to bias differentiation of stem cells to a lineage corresponding to sensory neurons by adding specific growth factors in a timed sequence, and by over expressing transcription factors in the pathway to sensory neurons. Clonal cell lines isolated under this Aim will serve as additional donor cell types to be studied in Aims 3 and 4. [unreadable] [unreadable]